1. Field of the Invention
The present invention relates to an optical semiconductor module and a method of manufacturing the same, particularly, to an optical semiconductor module adapted for the light transmission over a short distance and capable of realizing a stable optical coupling with a relatively simple structure.
2. Description of the Related Art
A drastic improvement in the operating speed has been achieved in the large scale integrated circuit because of the improvement in the performance of the electronic device such as a bipolar transistor or a field effect transistor. However, even if the operating speed inside the LSI has been achieved, the operating speed on the level of the printed circuit board to which the LSI is mounted is suppressed to a level lower than that inside the LSI, and the operating speed is further lowered on the rack level with the printed circuit board involved therein. The suppressed operating speed is derived from the transmission loss of the electric wiring accompanying the elevation of the operating frequency and from the increases in noise and in the electromagnetic interference. It is unavoidable for the operating speed to be suppressed to a low level because, in order to ensure the signal quality, the operating frequency is suppressed to a low level with increase in the length of the wiring. It follows that, in the electric wiring apparatus, the tendency that the mounting technology rather than the operating speed of the LSI controls the operating speed of the system is rendered stronger and stronger in recent years.
In view of the problems noted above in conjunction with the electric wiring apparatus, several optical wiring apparatuses for optically coupling different LSI s have been proposed as disclosed in, for example, Japanese Patent Disclosure (Kokai) No. 2000-347072. The optical wiring is scarcely dependent on the frequency in respect of, for example, the loss in the DC region and the AC region having the frequency exceeding 100 GHz, and the wiring path is substantially free from the electromagnetic interference or the electrical ground potential fluctuating noise. Such being the situation, it is possible to achieve easily the wiring of scores of Gbps. In order to realize the optical wiring of this kind between different LSI s, required is an optical semiconductor module having a simplified construction as disclosed in, for example, Japanese Patent Disclosure No. 2000-347072 quoted above. Also, a large number of optical transmission paths are required as the LSI wiring, and it is necessary for the optical semiconductor module to be manufactured at a very low cost.
In general, an image-forming lens, etc. are incorporated in an optical semiconductor module, and an optical guide coupling section has a connector structure. Such being the situation, it is difficult to miniaturize sufficiently the optical semiconductor module in many cases. On the other hand, in the optical semiconductor module disclosed in Japanese Patent Disclosure No. 2000-347072 quoted above, an optical transmission path such as an optical guide is coupled directly with a semiconductor device so as to form an integral structure. As a result, the miniaturization can be achieved relatively easily. However, the particular construction gives rise to several problems as pointed out below.
Specifically, in the optical semiconductor module disclosed in Japanese Patent Disclosure No. 2000-347072, an optical guide and the holding member thereof are formed integral, and a pattern electrode is formed on the holding member for mounting a semiconductor device to the integral structure noted above. It follows that it is necessary to perform the pattern depiction or the pattern transfer of the electrode in a very small edge section of the optical guide holding member. To be more specific, it is necessary to carry out the patterning having an accuracy of several microns under the state that a guide of several meters to scores of meters is kept mounted. It is practically difficult to manufacture the optical semiconductor module by this manufacturing method. However, it is absolutely necessary to employ the particular method in the case of employing at least an arrayed semiconductor device. It follows that it is substantially impossible to manufacture the optical semiconductor module on a mass production basis by the particular manufacturing method. Alternatively, in the case of employing the particular manufacturing method, the product yield is very low.
It should also be noted that, in the optical semiconductor module disclosed in Japanese Patent Disclosure No. 2000-347072 quoted above, the edge surface of the optical guide and the plane to which is mounted the semiconductor device are substantially on the same plane, with the result that the optical guide and the semiconductor device are positioned very close to each other so as to be coupled with each other. However, the surface emitting laser, which is a typical high speed optical signal source, is sensitive to the reflected light, i.e., the laser light emitted from the surface-emitting laser itself and reflected so as to be brought back to the surface-emitting laser. In other words, the reflected light from the optical fiber coupling section (near end reflection) is inherent in the surface-emitting laser. It follows that it is important to take measures against the reflected light at, for example, the light-emitting plane of the optical guide (far end reflection) in the optical semiconductor module disclosed in Japanese Patent Disclosure No. 2000-347072. The method of using an optical isolator provides the most reliable measure against the problem noted above. However, the optical isolator is highly costly. In addition, an additional problem is generated that the space for incorporating the optical isolator renders the module markedly bulky.
As another measure, it is conceivable to apply an antireflection coating to the edge surface of the optical guide or to apply an oblique processing to the edge surface of the optical guide. This method is certainly effective as a measure against the reflected light. However, in the prior art such as Japanese Patent Disclosure No. 2000-347072, the optical guide and the holding member are made integral so as to form a pattern electrode for the semiconductor device. As a result, it is also necessary for the no-reflection coating to be formed in a pattern. This requires an accurate pattern formation so as to give rise to the problem in terms of the productivity as in the technology described above.
It is also possible to moderate the influence given by the reflected light by setting appropriately the distance between the surface-emitting layer and the optical guide. To be more specific, if the distance between the surface-emitting laser and the optical guide is extremely long, the optical coupling is simply rendered weak so as to make the optical transmission itself difficult. It should be noted in this connection that, if the distance between the surface-emitting laser and the optical fiber is set appropriately, the optical coupling is certainly rendered low. However, the reflecting light is also diminished. It follows that the influence given by the reflected light can be considerably suppressed while maintaining the capability of the optical transmission.
However, in the optical semiconductor module disclosed in Japanese Patent Disclosure No. 2000-347072 quoted above, it is substantially difficult to control the distance between the semiconductor device and the edge of the optical fiber. Particularly, where the semiconductor device is apart from the edge of the optical fiber by about 100 μm, it is necessary to control the thickness of the spacer or to control the etch back of the optical fiber, with the result that the reproducibility is rendered highly poor.
What is most important is that the edge plane of the optical fiber is formed by the polishing when it comes to the optical semiconductor module disclosed in Japanese Patent Disclosure No. 2000-347072 so as to give rise to the problem that the manufacturing cost in respect of the edge plane of the optical fiber occupies a very high weight. In general, the polishing of the optical fiber including the mounting of the optical fiber to the polishing apparatus, the rough polishing, the intermediate polishing, and the finish polishing necessitates a long processing time exceeding several hours. It follows that the productivity is not improved. In addition, the cost reduction is limited.